The present invention concerns a liquid crystal display cell in which there are incorporated measuring and reference elements which, by means of an electronic circuit, make it possible to compensate for the variation in the contrast of the display in dependence on temperature. Such a cell can advantageously be used when use is made of a multiplexed addressing arrangement, as is the case for example in timepieces.
The low level of consumption of liquid crystal display cells and the fact that the operating voltages are low, of the order of three volts, means that such cells are being put to an increasing number of uses, in particular in portable devices.
However, as the number of items of information displayed increases, the number of connections for the cell also rises. This has unfavorable repercussions on the level of reliability, size and cost of the cell, making it more difficult for it to be used in watches. One way of reducing the number of connections on the cell, while retaining the same possible information modes an capacity, comprises using multiplexed addressing.
However, multiplexed addressing is less favorable from the point of view of distribution of the voltages among the various electrodes of a cell, than direct electrode-by-electrode addressing. This means that, in order to benefit from the reduced number of connections, the operating voltages of the cell must be suitably selected in dependence on the optical characteristics of the liquid crystal used in such a way that an item of information, for example a segment, can have only two states, visible and invisible. Now, the characteristics of the liquid crystals vary in dependence on temperature. This is the case in particular in regard to an important parameter, referred to as the optical threshold voltage, which is defined as being the value of the voltage applied to the electrodes of a segment, above which it begins to become visible. If the operating voltages of a cell are so selected that the display operates in the optimum manner at a certain temperature, at a different temperature the segments which should remain invisible may no longer be completely invisible, or segments which should remain visible may be visible but with an attenuated degree of contrast. In both cases, the result is a fall in the quality of the display with temperature, which can be to such an extent as to become unreadable.
One way of overcoming this difficulty would be to maintain the display at a constant temperature by a heating means but the consumption thereof would limit its uses and would exclude its being used in watches. Another way which is often used comprises employing a voltage divider formed by a resistor connected in series with semiconductor diodes. The voltage at the terminals of the diodes falls when the temperature rises, so it is possible to use it for supplying power to the cell. The level of compensation is not strict, but it is simple and its main disadvantage lies in the additional consumption of power due to the voltage divider.
Another way has been proposed in European patent application publication No. 0002920. This involves using an auxiliary capacitor which is incorporated in the cell, acting as a measuring element. The capacitor is formed by two electrodes which are disposed on the opposite main faces of the cell, having the liquid crystal as the dielectric. The value of the capacitance of the measuring capacitor depends inter alia on the temperature and the excitation voltage at which it is measured. Variation thereof in dependence on the above-mentioned voltage is highly non-linear, having a constant plateau for low voltages and a rising variation of parabolic type for higher voltages. The connection between the two curves precisely defines an electrical threshold voltage which depends on temperature. Now, at each temperature, the optical and electrical threshold voltages are practically equal as they correspond to the occurrence of the same phenomenon in the liquid crystal, namely the molecules turning from a direction perpendicular to the electrical field to a direction parallel thereto, in proportion to that field increasing. It should be noted however that the electrical phenomenon appears a little more quickly than the optical phenomenon, which causes a shift of about 0.1 volt between the electrical and optical threshold voltages. This is a small difference and it can be disregarded, although it is also possible to take account of it in a compensation circuit.
In accordance with the above-mentioned published application, thermal compensation for the display is achieved by controlling the operating voltages of the cell in dependence on the electrical threshold voltage, the latter varying with temperature in accordance with the same law as the optical threshold voltage. For that purpose, the impedance of the measuring capacitor of the cell is compared in a bridge to the impedance of a capacitor or another component serving as a reference, which is disposed outside the cell. If the two impedances are different, an error signal acts on an electronic circuit which alters the excitation voltage of the measuring capacitor so as to remove the unbalance. The resulting voltage corresponds to the electrical threshold voltage of the cell and the cell operating voltages can be directly derived therefrom. However, this form of compensation suffers from the disadvantage of requiring a reference element external to the cell, which is to be stable in respect of temperature. In addition, the capacitance of the measuring capacitor depends on the thickness of the display cell and the dielectric constant of the liquid crystal. As these two parameters may vary from one cell to another, the reference capacitor and the measuring capacitor of the cell must be individually paired, which is a major handicap in manufacture.
A cell which does not suffer from the above-indicated disadvantage is described in European patent application publication No. 0012479 and is shown in plan view in FIG. 1a and in section taken along line A--A in FIG. 1b of the accompanying drawings. It essentially comprises two glass plates 1 and 2, an insulating frame 3 for maintaining a constant spacing between the plates 1 and 2, and two polarisers 4 and 5, the polarization axes of which are parallel to the plates and perpendicular to each other in the described embodiment. Disposed on the inside faces of the plates 1 and 2 are transparent display electrodes, the combination of which reproduces the shape of the symbols to be displayed being in the present case four digits each formed by seven segments. The electrodes which are disposed on the rear plate 2 are often referred to as counter-electrodes. The output connections of the electrodes and their internal connections are not shown as they depend on the mode of controlling the cell. In addition, the inside faces of the plates 1 and 2 each carry an alignment layer intended to orient the molecules of the liquid crystal so as to be able to make use of the optical properties thereof. Finally, if reading and illumination of the display cell are from the same side, as is the case with a watch, a light reflector or diffuser 10 is placed behind the cell. Electrodes 6 and 8 forming a reference capacitor are arranged facing each other on the inside faces of the plates 1 and 2. A measuring capacitor is formed in a similar manner by means of electrodes 7 and 9. The areas of the electrodes 6 and 8 are equal to each other while being larger than the area common to the electrodes 7 and 9. The capacitance C.sub.R of the reference capacitor is therefore higher than the capacitance C.sub.M of the measuring capacitor. The variations in dependence on temperature of the voltage of the common point of the two capacitors are used to vary the operating voltages of the segments of the cell.
This cell suffers from the disadvantage that the excitation voltages of the measuring and reference capacitors must be different, the lower voltage being applied to the reference capacitor to cause it to operate in a region in which its characteristics are independent of temperature, which involves complication in the electronic circuit for supplying the voltages.